The present invention concerns improvements in or relating to information processing units which operate in a multiprogrammed fashion and wherein a microprogram control store comprises both a read-only storage portion and a read-write storage portion, each portion being organized in "pages" or sectors. Predetermined microprograms are stored permanently into the read-only storage portion. On the other hand, further microprograms may be introduced into the read-write portion of the control store during execution of a task on either a machine decision or a human decision. The human decision will mainly come from an operator's console to which a microprogramming error is signalled for a microprogram sector of the read-only storage portion of the unit. The machine decision may be motivated both by the necessity for recourse to an additional microprogramme which does not exist in the read-only storage portion of the control store and by the detection of an error in a sector of the read-only storage portion of the store. Any intervening decision will temporarily interrupt the execution of the task and introduce the requested additional or amended microprogram portion into an addressed sector of the read-write storage portion of the control store.
In such multi-microprogrammed units, it is conventional to provide an automatic organization for the progression of the addresses of the control store. Each control word read-out from the store includes a part pointing at the address sector of the next word to be read into the central store and, when needed, it further includes a part pointing a modification in the chaining of the store addresses. The control store is operatively associated with a logic unit. An input of the logic unit is fed with data from the read-out register, as for instance a data concerning the address of the stored word just read out. From such data from the read-out register and the logical unit (responsive to the data it has just received from the read-out register), a control store address is formed by an organization to the inputs of which said data are applied and which is provided with an output to the common address register of the control store. The thus formed new address then controls the next read-out of a control word from the store.